1. Field of the Invention
This invention relates generally to a magnetoresistive sensor, and more particularly to such a sensor based on extraordinary magnetoresistance (EMR).
2. Description of the Related Art
A magnetoresistive sensor based on extraordinary magnetoresistance (EMR) has been proposed as a read-head sensor for magnetic recording hard disk drives. Because the active region in the EMR sensor is formed of nonmagnetic semiconductor materials, the EMR sensor does not suffer from the problem of magnetic noise that exists in read-head sensors based on giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR), both of which use magnetic films in their active regions.
The EMR sensor is fabricated as a mesa comprising a semiconductor heterostructure on a substrate. A pair of voltage leads and a pair of current leads are formed on one side of the mesa in contact with the semiconductor active region and an electrically conductive metal shunt is formed on the other side of the semiconductor mesa in contact with active region. In the absence of an applied magnetic field, injected current through the current leads passes into the semiconductor active region and is shunted through the metal. When an applied magnetic field is present, current is deflected from the shunt and travels a longer distance through the semiconductor active region. Because the semiconductor is much more resistive, the electrical resistance of the device increases. The change in electrical resistance due to the applied magnetic field is detected across the voltage leads. EMR is described by T. Zhou et al., “Extraordinary magnetoresistance in externally shunted van der Pauw plates”, Appl. Phys. Lett., Vol. 78, No. 5, 29 Jan. 2001, pp. 667–669. An EMR sensor for read-head applications is described by S. A. Solin et al., “Nonmagnetic semiconductors as read-head sensors for ultra-high-density magnetic recording”, Appl. Phys. Lett., Vol. 80, No. 21, 27 May 2002, pp. 4012–4014.
The EMR sensor as described in the prior art is difficult to fabricate because the lithography for the shunt and leads must be done on a nonplanar surface, i.e, the sides of the mesa. Moreover the contact area giving rise to the contact resistance is only defined by the thickness of the EMR active region and the lead width. For small sensors this contact area is small resulting in a high contact resistance and a loss in signal-to noise ratio. In addition, chemical changes of the active region by exposure to the environment can negatively impact the electrical properties of the active region.
What is needed is a planar EMR sensor with a shunt and leads that are patterned on top of the sensor and extend perpendicularly downward to provide electrical contact with the EMR active region to thereby improve the fabrication process and solve the problems of high lead contact resistance and exposure of the EMR active region to the environment.